In Korea, more than 3000 missing children are reported in a year. According to the statistics data of a year 2005 from a national police agency, children were generally lost around their home as well as public places with crowded such as amusement parks, malls, and national parks. The missing child problem is no more a personal problem. It became a social problem that all parents must consider. Children cannot remember a home telephone number or a home address if they lost their parent although they memorize the home telephone number or the home address before. Senior citizens or mentally disabled persons may have the same problem if they lost their guardians. Such a missing children problem is not only a problem of Korea. It has been addressed as a worldwide problem. Therefore, there have been many researches in progress for overcoming the missing children problem.
Also, crimes related to kidnapping, violence, bandits, and homicides have increased day by day.
In order to prevent the missing children or the crimes, it is necessary to monitor a target object to protect through real-time location determination scheme. Here, the target object to protect may be a child, a mentally disabled person, a pet, a valuable product, and an art. In case of the missing child, children would have more mental damages and may become more frightened as time goes on. It is better to find the missing children as soon as possible. In order to prevent kidnapping and missing child, it is necessary to provide a missing child preventing service using a ubiquitous computing system. Therefore, the missing child preventing service has been developed using high-tech such as RFID, GPS network, CCTV, and mobile communication. The effectiveness of the missing child prevention service can be improved through an accurate location tracking technology.
Hereinafter, the shortcoming of location tracking technologies according to the related art will be described. The wireless communication technology is classified into a wide range wireless mobile communication technology for providing a wireless communication service in a wide area and a short range wireless mobile communication technology for providing a wireless communication service in a small area such as home, an office, a factory, a warehouse, and a special complex.
A conventional short range wireless communication technology enabled voice communication or simple and small data transmission through a two way radio. According to the development of a digital communication technology and a semiconductor technology, various short range wireless communication solutions have been developed and introduced for transmitting various wireless data, wireless-controlling devices, and transmitting multimedia data such as voice, audio, and video. Particularly, many wireless communication solutions for determining the locations of various objects in a short range communication network have been developed.
The location determination technology for determining locations of mobile objects has being receiving attraction and there have been many researches in progress because the location determination technology may be used not only for military purpose but also civil purposes such as preventions of mission child, distribution management, business support, supervision of rescuers in emergency situation, and providence of driving information.
In the location determination technology according to the related art, a global positioning system or the location information in base stations in a mobile communication network were generally use.
Since the GPS provides a stable service using fixed satellites having a wide signal range, the GPS has been widely used to determine locations of mobile objects in outdoor. However, the GPS has shortcomings as follows. That is, the GPS has a limitation of using the satellites and has high power consumption. Also, a time to first fix (TTFF) is very long, and the accuracy thereof may deteriorate in an urban area and a shadow area due to multipath and insufficiency of visual satellites.
As described above, it is impossible to use a GPS based location determination system in an indoor place and a densely packed urban area. Also, the GPS based location determination system may provide comparative good performance in wide area but may provide inaccurate result in a small area, such as an area of several tens square meters.
In order to overcome such shortcomings, Korea Patent Publication No. 0351962 entitled “Navigation system using pseudo satellite” disclosed a method for determining a location a mobile object in an indoor place by modifying a location determination system according to the related art. However, this method has a problem of large signal intensity variation in a short range.
Meanwhile, various location awareness technologies using ultrasonic wave were introduced. For example, the bat ultrasonic location system was introduced as an active bat system from AT&T Laboratories Cambridge. The ultrasonic location system radiates ultrasonic wave to a mobile object. Due to the interference characteristics of the ultrasonic wave, it is difficult to simultaneously transmit a plurality of ultrasonic waves. In the ultrasonic location determination system, a mobile object radiates the ultrasonic wave and a plurality of receivers measure an arrival time. However, the ultrasonic based location determination method is applicable to one mobile object.
Korea Patent Publication No. 2003-0093387 entitled “ultrasonic sensor system for estimating location and direction of mobile object” introduced a method of receiving ultrasonic wave in a mobile object. Since the mobile object receives the ultrasonic wave, it is impossible to independently operate each of mobile objects. Particularly, a predetermined rule is required to operate due to interference of ultrasonic wave when a plurality of mobile objects are present.
A method for measuring a distance using ultrasonic wave through maximum amplitude in U.S. Pat. No. 5,793,704 issued to Freger and entitled “METHOD AND DEVICE FOR ULTRASONIC RANGING”. However, this method is very complicated and has limited performance.
Various approaches have been introduced for determining a location of a mobile object using wired/wireless mobile communication. However, the accuracy thereof abruptly varies according to a size of a base station cell and a measuring method due to the measurement error, for example, from about 500 meters to several kilometers according to the location of a base station. For accurate measurement, several methods were introduced using radio frequency identification (RFID).
The RFID technology was introduced to identify an object. However, the RFID technology has been advanced to a location awareness technology. Particularly, the RFID has advantages of determining a location and transmitting and receiving information through a sensor network. However, the RFID has been used for location determination in an indoor space. The RFID has not been used for location determination in an outdoor space.
As an example of using RFID for location determination, a system Cricket was introduced. In the Cricket, a Beacon is installed at a ceiling and radiates an ultrasonic wave and a RF signal. A Listener senses the ultrasonic wave or the RF signal transmitted from the Beacon. Then, a distance from the Listener to the Beacon is measured using the difference of arrival times of the ultrasonic wave and the RF signal. The Cricket accurately determines the location of a mobile object because the Cricket includes an algorithm for preventing the collision of Beacons and another algorithm for estimating a location using a kalman filter. However, the Cricket also has shortcomings. That is, a Beacon must be installed at every 16 sqft (about 1.32232 m2) because of a short ultrasonic range. If visual is not obtained due to obstacles, it is difficult to determine the location of a mobile object.
A location identification based on dynamic active RFID calibration (LANDMARK) was introduced. The LANDMARK applies a location reference point to a RFID tag in order to calibrate a location value. The LANDMARK reduces the number of readers required for indoor measurement and sustains the reliable accuracy through dynamically updating information about a reference point. However, the LANDMARK cannot accurately determine locations when obstacles are present.
Since the RFID technology was designed for indoor places, it is not suitable for a mobile object that travels outdoor places at a high speed.